The increasing request
for energy quality and, thus, insulation system reliability, associated
with significant steps in the manufacturing of new insulating materials,
are pushing strongly the research on new techniques for the diagnosis of
insulation ageing.
Space charge observation is becoming the most widely used technique to
evaluate polymeric materials for dc-insulation applications, particularly
high-voltage cables.
It is already well assessed in fact, that the presence of space
charges is the main problem causing premature failure of high-voltage dc
polymeric cables and, indeed, the main reason preventing from a rapid
diffusion of such kind of cables [1-4]. Moreover, it has been shown that insulation
degradation under service stresses can be diagnosed by space charge
measurements. However, quantities are
still lacking that can help to summarize and interpret the huge amount of
data resulting from space charge measurements, and that are also
associated with the electrical performance of the insulation.
The pulsed electroacoustic analysis (PEA) can be used for space charge
measurements under dc or ac fields.
The PEA method is a non-destructive technique for profiling space charge
accumulation in polymeric materials. The method was first proposed by T.
Takada et al. in 1985 [5].
A series of high-voltage pulses of very short time length is applied to an
insulation specimen interposed between two electrodes. Each pulse produces
an electric force displacing internal charges and generating pulsed
acoustic pressure waves in correspondence of each charge layer in excess
with respect to neutrality. The resultant pressure pulse is detected by a
piezoelectric transducer, so that the charge distribution in the specimen
under test can be obtained from the output voltage of the transducer. The
analysis of space-charge profiles is restricted to one dimension.

LIMAT can perform PEA measurements on:

flat specimens of insulating material

high voltage cables

enameled wires for machine windings (innovative
system compoletely developed at LIMAT)

PEA measurement
system for flat specimens

The most
simple PEA system is able to detect the inner space charge in a polymeric
flat specimen with thickness up to 0.5 mm.

A complete
PEA system (Fig. 1) is composed by:

a high
voltage generator (AC or DC)

a voltage
pulse generator

the PEA
cell

a digital
oscilloscope

a
personal computer

a GPIB
card to interface computer and oscilloscope.

Figure
1 shows more in detail the components of the PEA cell (note, in
particularly, the presence of a decoupling capacitor, C, between high
voltage supply and pulse generator circuits).

The
resistor, R, is used in order to reduce the discharging current in the
case of specimen breakdown.

A film of
semiconductive material is generally interposed between the upper brass
electrode and the specimen, in order to reduce the difference of sonic
impedance between two different materials and to allow a good propagation
of the pressure wave generated by voltage pulse.

The specimen is fed by an high-voltage power supply. A
voltage pulse, with an amplitude of 500 V, 10 ns width, is applied to the
specimen through a coupling capacitor. The pressure wave
propagating through the insulation down to the lower aluminum ground
electrode reaches the piezoelectric transducer. The
piezoelectric transducer (PVDF) generates a voltage signal (PEA output
signal) proportional to the pressure wave propagating through it. The weak
voltage signal generated by the PVDF is then amplified by two large-band
amplifiers, is sent to an oscilloscope and recorded by a personal
computer, connected with the oscilloscope through an IEEE-488 bus. A PMMA
adsorber is located under the piezoelectric transducer in order to avoid
reflections which can disturb the PEA output signal.

Fig.1: PEA
cell scheme for flat specimens.

PEA measurement
system for cables

This PEA
System has been developed to measure directly space charge profiles on
Cables [6]. The system has been deviced for quality control and diagnostic
purposes. The main feature is the sensibility of the system, required to
achive consistent measurements on thick specimens. Used for
factory and on site (off line) measurements, the system detects quantities
related to water tree and insulation degradation processes, providing a
bulk diagnosis of the cable.

The system
for space charge measurements on cables is realized according to the idea
reported in [7]. As sketched in Fig. 2, it consists of a HVDC generator, a
pulse generator and the measurement system (amplifier and piezoelectric
sensor). A voltage pulse is applied between outer semicon and ground,
unlike the conventional PEA method for flat specimens. A decoupling
capacitor is not needed because the cable itself is used for this purpose.

Fig.2: PEA
cell for cables.

PEA measurement
system for enamelled wires

The measurement of space charges through the
PEA system developed for this application constitutes a new useful tool to
evaluate and compare the electrical properties of the enameled wires used
for motor windings. The comparison of different materials through space
charge measurements shows that significant
differences regarding space charge trapping properties can be introduced
modifying enameled wire insulation.

This PEA System has been developed to measure directly
space charge profiles on enameled wire insulation. The main feature is the
high spatial resolution necessary for specimens with thickness between 15
and 50 μm.

A
scheme of the new PEA system developed with the purpose to measure space
charge accumulation on enameled wires is reported in Figs. 3 and 4 [8]. This
system is realized starting from the PEA version for power cable described
in the previous section [6].

The
circuit diagram is reported in Fig. 3. A front
view of the test cell realized is shown in Fig. 4. The enameled wire under
test can be correctly positioned on the aluminum ground plate through the
central fixing screw on the support structure (note that the dimensions
are not in scale).

The
main difference with respect to the other PEA systems developed in the
past is the specimen to be tested. Till now systems have been developed
for flat specimens or thick power cables. The PEA measurement on enameled
wires presents specific features and difficulties. Due to the small size
of the wire (diameter of about 1 mm) the contact with the aluminum ground
plate may be unsatisfactory. Moreover, the small thickness of the enameled
insulation (30-40 mm)
requires the use of very short pulse and very thin transducer, in order to
achieve a good spatial resolution needed for such thin insulation.

Fig. 3 Circuit diagram of the PEA measuring cell

Fig.4: Front view of the PEA measuring cell

Exemples of PEA
measurement results

Indipendently of the measurement system used (flat
specimen, calbes, etc.), one of the most important PEA results consists of
profiles of space charge accumulated in the insulation bulk, from which
the inner electric field distribution is obtaineable.
The figure 5 shows examples of space charge accumulation profiles for an
enamelled wire during poling (Fig. 5A) and after voltage removal (Fig. 5B). After 20 s from the voltage application
(Fig. 5A) we see
a flat zone between the two peaks representing the electrodes, which means
that injected charges have not reached yet the insulation bulk. One hour
after, a large amount of negative charge, injected by the electrode,
accumulates in the insulation bulk, which causes the apparent shifting of
the negative electrode peak visible in Fig. 5A, as well as the increase of
anode charge. This is, again, confirmed by the profile under volt-off
reported in Fig. 5B, which shows clearly the presence of bulk negative
space charge, which decreses as the time elapses.

Fig. 5: Space charge profiles at two different poling
times (A) and space charge profiles at two depolarization times (B).
Enameled wire insulation.

Another
important information that can be carried out from PEA measurement is the
colored pattern relevant to the spatio-temporal evolution of the net
charge during polarization and depolarization at different fields (Fig. 6). Each
representation has three dimensions: thickness, time and charge. Thickness
is the Y axis (cathode and anode are indicated), time is the X axis (non
linear scale), and the 3rd dimension is charge, represented by a color
scale. All the
values exceeding the maximum level, positive or negative, are associated
with the color white. This can happen, in particular, at the electrodes,
where the applied voltage leads to highest values of charges (capacitive).

Charge packets during polarisation and trapped
charge decay during depolarisation are observable in Fig. 6.

Fig. 6: Charge pattern windows. The color scale is represented on the right side
of the window. For this example the maximum positive value is 50 C/m3 and
the maximum negative value is -50 C/m3.